U.S. Pat. No. 5,389,584 (Aitken et al.) describes stable glasses that exhibit excellent transmission far into the infrared region of the spectrum. These glasses consist essentially of, expressed in terms of mole % on the sulphide basis, 55-95% GeS.sub.2, 2-40% As.sub.2 S.sub.3, 0.01-20% Ga.sub.2 S.sub.3, and/or In.sub.2 S.sub.3 and 0-10% MS.sub.x wherein M is at least one modifying cation selected from the group consisting of Li, Na, K, Ag, Tl, Ca, Sr, Ba, Cd, Hg, Sn, Pb, Al, Sb, Y and rare earth metals of the lanthanide group, 0-20% Cl and/or F, 0-5% Se, and wherein the S and/or Se content can vary between 85 and 125% of the stoichiometric value.
When glasses having compositions within these ranges were doped with praseodymium ions (Pr.sup.3+), it was found that the glasses exhibited strong .sup.1 G.sub.4 fluorescence at 1.3 .mu.m with a lifetime (.tau.) value of at least 300 .mu.sec. However, this was only achieved if an effective amount of gallium and/or indium were present in the glass composition. These components served to disperse the Pr.sup.3+ dopant, thereby avoiding the deleterious effect of concentration quenching. Otherwise, in a binary, arsenic germanium sulphide glass doped with Pr.sup.3+ ions, the latter dopant was completely clustered and, therefore, the glass exhibited very weak fluorescence at 1.3 .mu.m.
Such co-doped glasses have been shown to be particularly useful in the production of optical devices used in telecommunication. These devices include amplifiers, upconverters and lasers. The use is based on the fact that a rare earth dopant, such as the Pr.sup.3+ ion, is well dispersed in a glass containing gallium and/or indium ions as co-dopants. As a consequence, fluorescence emanating from the rare earth metal dopant is not degraded by concentration quenching, or by similar, non-radiative, quenching processes.
The invention of the--584 patent was directed at improving the thermal stability of known, gallium sulphide glasses. Thermal stability is evidenced by the difference between the temperature at the onset of crystallization in a glass and the glass transition temperature (T.sub.x -T.sub.g). At the same time, the desirable transmission characteristics, and other properties, of the gallium sulphide glasses were maintained.
The invention, on which the patent was based, derived from two fundamental discoveries. First, it was found that increased concentrations of arsenic in gallium, germanium sulphide glasses imparted enhanced thermal stability to the glasses. Second, it was found that the presence of gallium (Ga) and/or indium (In) in the glass eliminated rare earth clustering, thereby assuring, in the case of Pr doping, that the fluorescence at 1300 nm was not quenched.
The present invention is based on the discovery of a co-dopant, other than Ga and/or In, that is equally as effective in dispersing a rare earth metal dopant. At the same time, other important advantages are effected.
It is a basic purpose, then, of the present invention, to provide such alternative co-dopant for the rare earth metal doped glasses of the--584 patent, while retaining the properties of those glasses.
It is a further purpose to provide an improved, co-doped glass for optical devices used in telecommunications.
It is a still further purpose to provide a co-dopant that will result in at least some increase in the phonon energy level that can be generated in the glass.
It is another purpose to provide a means of dispersing a rare earth metal dopant in a glass, thereby enhancing the effectiveness of the glass.